Ayote Cucurbita spp

Clean plants

In order to keep dose rates in nuclear power plants low, it is important to keep the quantity of corrosion products that could be brought into the reactor core and subsequently activated to the lowest possible level, in other words, to keep the nuclear reactor “clean” from the design and construction stages. Such efforts will also contribute to improvements in the integrity of system piping and devices.

Japan: Measures for clean plants (Onagawa, Kashiwazaki Kariwa and Hamaoka NPSs)

Anti-pollution measures have been taken at nuclear power stations in Japan, including Onagawa, Kashiwazaki Kariwa, and Hamaoka. The Onagawa NPS has been working, since the construction of Unit 1, on the realisation of a highly reliable clean plant with the least exposure possible through various countermeasures developed under the “Clean Plant Campaign.” Of the countermeasures identified, “crud reduction measures” have been positioned as a top priority item for the realisation of a clean plant and have been thoroughly implemented.

Crud reduction measures can be classified as “crud generation control” and “crud removal”. Measures implemented at Onagawa NPS are characterised by the “storage management” of the primary piping, which has been thoroughly implemented as part of its crud generation control measures, in addition to the measures implemented for crud removal. Since the system conditions vary with the characteristics of the construction/test/trial run periods, Onagawa NPS selected the most appropriate storage method for its particular conditions. For example, generation of corrosion products inside the piping and devices was inhibited as much as possible by adopting measures such as the wet lay-up method with hydrazine (avoiding to the maximum extent possible wet lay-up with only pure water) and the dry lay-up method with dehumidified air, according to the conditions of the system piping. In addition, a system was developed to monitor appropriate storage conditions. As a result of such countermeasures, the Onagawa NPS was able to restrict the quantity of crud brought into the nuclear reactor to an extremely low level. Today, Onagawa NPS maintains the highest class in regard to extremely low dose rate through the combined implementation of other dose reduction measures.

Adoption of low cobalt materials

The sources of cobalt, which are chief sources of occupational radiation exposure, are stainless steel containing cobalt as an impurity, nickel-based alloys, and Stellite (a cobalt-based alloy). Since stainless steel is used in large quantities as a major structural material for the primary system, its cobalt content is very important. Stellite, chiefly composed of cobalt, generates Co-60 through the activation of its corrosion products, and is therefore a major source of occupational radiation exposure. In order to control the generation of corrosion products, it is necessary to use materials with excellent corrosion resistance and less cobalt content. Efforts have been made worldwide to reduce the Co content of materials, and many plants have replaced such materials with those of lower cobalt content in applications such as pin rollers in the control rod system, heating tubes for feed water heaters (stainless steel), fuel springs (Inconel), some stainless steel materials for the nuclear reactor system, etc. For example, most of the Stellite used inside the reactor vessel of the latest KWU nuclear plant in Germany (KONVOI) has been replaced with cobalt-free materials to significantly reduce the dose rate of the primary coolant piping system.

The use of low cobalt materials needs to be taken into consideration from the design stage, and is especially important for materials with large surface areas placed in corrosion-prone environments. Low cobalt materials and alternative materials for Stellite have been developed. For example, EPRI has developed hardfacing NOREMTM _{iron-based alloys as a substitute for Stellite used for valves in} nuclear power plants. Their use will enable the reduction of cobalt radiation sources that contribute to occupational exposure.

Surface treatment within piping

The accumulation of radioactive materials within piping can be prevented by forming in advance an oxide film on the surface of the stainless steel. Reducing the adhesion surface area by smoothing the material surface is also effective for the preventing accumulation of contaminants. Crud adhesion can be prevented by electropolishing the inside of stainless steel piping following the mechanical polishing undertaken during nuclear power plant construction or replacement of piping or devices. For

example, valves and piping were mechanically polished and electropolished for the restart of Browns Ferry Unit 1 in the United States. In France, electropolishing of steam generator channel heads is now performed for steam generators replaced in old plants as well as for the new reactor EPR.

Installation of filters (a) Hollow-fibre filters

Hollow-fibre filters are composed of an extremely thin hollow-fibre polymer material of approximately hundreds of μm to 1 mm in outer diameter with numerous micropores of approximately 0.1 μm in diameter. Characterised by an extremely large filtration area per unit volume (10 to 100 times larger than those of other membrane filters), the hollow-fibre filter can reduce the size of filtration equipment to less than other filter systems.

This type of filter was first introduced in Japan in 1986 within the condensate system of a BWR, with the objective of shortening the pre-startup cleaning time. Because of its excellent performance, it was then also used for purifying the partial condensate during power operation. Later, these filters were applied to the condensate system of many Japanese BWRs and PWRs. In terms of its cleaning performance in operating BWRs, while the filter inlet Fe crud concentration ranges from several thousand ppb (pre-startup cleaning stage) to several tens ppb (power operation stage), the outlet filtered Fe crud concentration is constant at 0.1 ppb or lower. In plants that came into service after 1993, a complete condensate purification system using this type of filtration was implemented with the chief objective of reducing the feed water Fe crud levels. The introduction of this system resulted in reduction of the feed water Fe crud level to 0.1 ppb or lower and thus significantly contributed to the exposure reduction of workers.

(b) Pleated filters

A pleated filter is a membrane filter in a pleated form, resulting in an increased filtration area. Pleated filters were first used in the early 1990s chiefly in the United States and have been improved since then. Since the pleated filter elements are burnable, and contain no powdered resins, they contribute to the reduction of radioactive wastes. In addition, pleated filters are characterised by their compatibility with the filter housing for precoat-type filter elements. They can therefore be used in existing plant systems simply by changing the element without modifying the filter housing.